1. Field of the Invention
The invention relates to a switching network for an asynchronous time-division multiplex transmission system including a first stage which has a plurality of switching blocks which, over auxiliary lines, are supplied with cells or packets of digital data and routing information intended for various trunk lines and are coupled to at least one switching block of a second stage.
2. Description of the Related Art
In an asynchronous time-division multiplex transmission system, useful information, for example, television, video or audio signals, is transmitted in blocks of fixed lengths through systems for digital signal transmission. A block of a fixed length is understood to be a cell that comprises a predetermined number of bytes (for example, 53 bytes). Such a cell may be transmitted serially, i.e. bit by bit, or in parallel, i.e. byte by byte through the digital systems. Each cell comprises a header field, for example, 5 bytes in length and an information field, for example, 48 bytes in length which contains the useful information. Such a header field contains a routing identification code, error detection data (header field error block) and further control information signals. A routing identification code is understood to be routing information and channel identification codes. The routing information also termed tag is added in specific transmission arrangements in the time-division multiplex transmission system and contains an identification of the sub-target of the cell in the transmission arrangement. It may be added to the header field of the cell or added to the sub-target over a further channel. The channel identification code, also termed VCI (virtual channel identifier), comprises the information about the target or sub-target of the cell in the asynchronous time-division multiplex transmission system. On the basis of the virtual channel identifier a virtual channel is made available for the transmission of a cell. Habitually, a VCI is changed each time after an exchange has been reached.
Cells are consecutively allocated to specific time slots. The duration of such a time slot depends on the transmission component clock frequency used as a basis. If no useful information is available, dummy cells are transmitted in such a time slot, i.e. cells without useful information. Cells containing useful information are referenced useful cells.
When cells are transmitted among subscribers, the cells pass through switching networks in which paths are realised on the basis of the tag. Such a switching network which is connected to a plurality of auxiliary lines and trunk lines, consists of a plurality of switching blocks. Such a switching block which has a plurality of input and output lines consists of various switching elements. A switching element is connected to a number of input lines and one output line. In a switching element the cells from an input line are transferred to one output line. When cells to be transmitted over one output line and coming from various auxiliary lines arrive during a time slot, they are further processed in a specific order.
In the journal of "Telcom Report", the article entitled "Mit ATM zur bitvariablen Kommunikation", No. 3, 1990, Vol. 1, pp. 4 to 7, describes an asynchronous time-division communication system which comprises a switching network for switching cells. Various subscriber units, for example, a videotape recorder, a personal computer and a switch controller are connected to the switching network by means of trunk line groups.